Two SHIV challenges have been studied in this project. The first involved 12 previously immunized animals. Six had been vaccinated with vaccinia virus expressing HIV-1LAI gp160 and subsequently boosted with purified subunit gp160 produced by vaccinia virus-infected AGMK cells. The other six served as mock-vaccinated controls, having been given vaccinia virus alone and boosted with IFA. Pre-virus challenge evaluation of the animals revealed serological responses similar to those achieved in humans immunized with various env preparations. The macaques were challenged with 25 tissue culture infectious doses (TCID50) of SHIVIIIB. The six mock-vaccinated animals became persistently viremic by virus culture and RNA PCR. In contrast, virus was recovered intermittently from one of the six vaccinated animals and the RNA PCR revealed virus levels several logs lower in the vaccinated animals. The animals will continue to be monitored and will be rechallenged with a more virulent SHIV. In the second challenge experiment, 22 M. nemestrina divided among 7 experimental vaccine and control groups also were challenged with SHIVIIIB. The results supported those in the first challenge experiment, that vaccinia env prime and env boost afforded the greatest protection against challenge. Two macaques, one from each control group, experienced spontaneous CD4 decline. To determine whether virus from these macaques had become more pathogenic, we gave whole-blood transfusions to two naive animals. The recipient macaques experienced a rapid and irreversible CD4 decline. Virus was recovered from the transfusion recipients and stocks were prepared. To confirm that the stocks were infectious in vivo, we inoculated two additional macaques with 1 ml of undiluted virus. These animals also showed CD4 depletion. The stock will be titered in additional macaques to determine the macaque infectious dose.